Leela, do you think you or his family could talk him into seeing, first, a regular doctor who could give him a physical?  If the doctor thinks it's depression (and he may need input from family to help make that diagnosis), he can either try giving him medication or, preferably, referring him to a psychiatrist or psychologist for an evaluation.  The time to get him to agree to seeing anyone is right after he's had one of his angry explosions and he's feeling remorse.  If he seems ill enough, or suicidal, someone could take him to a hospital emergency room or even call 911.

I'm sorry you're having to go through this, but know that it's not uncommon for people who are acutely depressed to push people away, usually the ones they are closest to.  This is partly due to feeling guilty for being depressed or not behaving the way they  know they should, so shame keeps them from getting help.  Maybe you and his family could also arrange an "intervention" where you calmly sit down with him and tell him how you feel about him, your concern, and what you observe.  Then ask him to agree to get help.  If he's an adult, there's nothing you can do if he doesn't want help except to call 911 if he's actually threatening himself or others.

Hopefully, he'll see that people do care about him.  If he can get through this crisis, he will realize it afterward, for certain.  I know it's really hard.  You might also think about talking to a psychologist yourself for some support and feedback on how to handle this.  I wish you all the best and hope you'll keep us posted.